1. Field of the Invention
This invention relates to a power transmission device constituting a power transmission path.
2. Background Art
As an example of such a kind of power transmission device, there is one as shown in FIG. 6. Such an example has a shaft 101 and a rotation body 102, both constituting a power transmission path. A thread surface 101a is formed at the outer peripheral surface of the tip end of the shaft 101. The rotation body 102 has a cylindrical portion 103 to be fitted on the outer surface of the shaft 101. A thread surface 103a to be engaged with the thread surface 101a of the shaft 101 is formed on the inner peripheral surface of the cylindrical portion 103. A seating face 101b is provided at the position adjacent to the thread surface 101a of the outer peripheral surface of the shaft 101 so that the rotation body 102 abuts against the seating face 101b thereby to restrict the screwing movement of the shaft 101 in an axial direction shown by L.
Thus, due to the screwing engagement of the shaft 101 with the rotation body 102, the rotation body 102 is pushed at the abutment surface 103b thereof as the tip end surface of the cylindrical portion 103 against the seating face 101b of the shaft 101, whereby the shaft 101 and the rotation body 102 are fastened and fixed to each other so as to be able to transmit power therebetween. That is, the seating face 101b of the shaft 101 and the abutment surface 103b of the rotation body 102 serve as power transmission surfaces between the shaft 101 and the rotation body 102.
However, in order to perform the fastening and fixing operation surely between the shaft 101 and the rotation body 102, in other words, to surely transmit power between the shaft 101 and the rotation body 102, it is required to secure a wide contact area between the seating face 101b and the abutment surface 103b. However, each of the seating face 101b and the abutment surface 103b, is formed by a surface perpendicular to the shaft line L of the shaft 101. Thus, particularly, the widening of the seating face 101b directly results in the enlargement of the diameter of the shaft 101.
When the shaft 101 is the driving shaft of a fluid-machine, the diameter of the driving shaft 101 greatly influences on various kinds of members operatively coupled to the driving shaft and so on a housing for containing the various kinds of members. Accordingly, the enlargement of the diameter of the driving shaft 101 results in the enlargement of the axial size of the fluid machine.